A
meteoroid is a
sand- to
boulder-sized particle of debris in the
Solar System. The visible path of a
meteoroid that enters
Earth's (or another
body's)
atmosphere is called a
meteor. If a meteor reaches the ground
and survives impact, then it is called a
meteorite. Many meteors appearing
seconds or minutes apart are called a
meteor shower. The root word
meteor
comes from the
Greek
meteōros, meaning "high in the air". A meteor is also
commonly called a
shooting
star and
falling star.
Overview
Meteoroid
The current official definition of a meteoroid from the
International Astronomical
Union is "a solid object moving in interplanetary space, of a
size considerably smaller than an
asteroid
and considerably larger than an atom." The
Royal Astronomical Society has
proposed a new definition where a meteoroid is between 100
µm and 10 m across.The
NEO definition includes larger objects, up
to 50 m in diameter, in this category. Very small meteoroids are
known as
micrometeoroids (see also
interplanetary dust).
The composition of meteoroids can be determined as they pass
through Earth's atmosphere from their trajectories and the light
spectra of the resulting meteor. Their effects on radio signals
also yield information, especially useful for daytime meteors which
are otherwise very difficult to observe. From these trajectory
measurements, meteoroids have been found to have many different
orbits, some clustering in streams (see
Meteor showers) often associated with a
parent
comet, others apparently sporadic.
Debris from meteoroid streams may eventually be scattered into
other orbits. The light spectra, combined with trajectory and light
curve measurements, have yielded various compositions and
densities, ranging from fragile snowball-like objects with density
about a quarter that of ice, to nickel-iron rich dense rocks.
Meteoroids travel around the sun in a variety of orbits and at
various velocities. The fastest ones move at about 26 miles per
second (42 kilometers per second) through space in the vicinity of
Earth's orbit. The earth travels at about 18 miles per second (29
kilometers per second). Thus, when meteoroids meet the Earth's
atmosphere head-on (which would only occur if the meteor were in a
retrograde orbit), the combined
speed may reach about 44 miles per second (71 kilometers per
second).
Meteor
- See also Hydrometeor.
A
meteor is the visible streak of light that
occurs when a meteoroid enters the Earth's atmosphere. Meteors
typically occur in the
mesosphere, and
most range in altitude from 75 km to 100 km. Millions of
meteors occur in the Earth's atmosphere every day. Most meteoroids
that cause meteors are about the size of a pebble. They become
visible between about 40 and 75 miles (65 and 120 kilometers) above
the earth. They disintegrate at altitudes of 30 to 60 miles (50 to
95 kilometers). Meteors have roughly a fifty percent chance of a
daylight (or near daylight) collision with the Earth as the Earth
orbits in the direction of roughly west at noon. Most meteors are,
however, observed at night as low light conditions allow fainter
meteors to be observed.
For bodies with a size scale larger than the atmospheric
mean free path (10 cm to several metres)
the visibility is due to the atmospheric ram pressure (not
friction!) that heats the meteoroid so that it glows and creates a
shining trail of gases and melted meteoroid particles. The gases
include vaporized meteoroid material and atmospheric gases that
heat up when the meteoroid passes through the atmosphere. Most
meteors glow for about a second. A relatively small percentage of
meteoroids hit the Earth's atmosphere and then pass out again:
these are termed
Earth-grazing fireballs.
Meteors may occur in
showers, which
arise when the Earth passes through a trail of debris left by a
comet, or as "random" or "sporadic" meteors, not associated with a
specific single cause. A number of specific meteors have been
observed, largely by members of the public and largely by accident,
but with enough detail that orbits of the incoming meteors or
meteorites have been calculated. All of them came from orbits from
the vicinity of the
asteroid
belt.
Fireball
A
fireball is a brighter-than-usual meteor. The
International
Astronomical Union defines a fireball as "a meteor brighter
than any of the planets" (
magnitude -4 or greater). The
International Meteor
Organization (an amateur organization that studies meteors) has
a more rigid definition. It defines a fireball as a meteor that
would have a magnitude of -3 or brighter if seen at
zenith. This definition corrects for the greater
distance between an observer and a meteor near the horizon. For
example, a meteor of magnitude -1 at 5 degrees above the horizon
would be classified as a fireball because if the observer had been
directly below the meteor it would have appeared as magnitude
-6.
Bolide
The word
bolide comes from the
Greek βολις, (
bolis) which can mean
a missile or
to flash. The IAU has no official
definition of bolide and generally considers the term synonymous
with fireball. The bolide term is generally used for fireballs
reaching
magnitude -14 or
brighter.The term is more often used among
geologists than
astronomers where it means a very large impactor.
For example, the
USGS uses the term to mean a
generic large crater-forming projectile
"to imply that we do
not know the precise nature of the impacting body ... whether it is
a rocky or metallic asteroid, or an icy comet, for example".
Astronomers tend to use the term to mean an exceptionally bright
fireball, particularly one that explodes (sometimes called a
detonating fireball).
Superbolide
If the
magnitude of a bolide
reaches -17 or brighter it is known as a
superbolide.
Meteorite
A
meteorite is a portion of a meteoroid or
asteroid that survives its passage through the atmosphere and
impact with the ground without being destroyed. Meteorites are
sometimes, but not always, found in association with hypervelocity
impact craters; during energetic
collisions, the entire impactor may be vaporized, leaving no
meteorites.
Tektite

Two tektites.
Molten terrestrial material "splashed" from a meteorite impact
crater can cool and solidify into an object known as a
tektite. These are often mistaken for
meteorites.
Meteoric dust
Most meteoroids are destroyed when they enter the atmosphere. The
left-over debris is called
meteoric dust or just
meteor dust. Meteor dust particles can persist in the atmosphere
for up to several months. These particles might affect climate,
both by scattering electromagnetic radiation and by catalyzing
chemical reactions in the upper atmosphere.
Ionization trails
During the entry of a meteoroid or asteroid into the
upper atmosphere, an
ionization
trail is created, where the molecules in the upper
atmosphere are
ionized by the passage of
the meteor. Such ionization trails can last up to 45 minutes at a
time. Small,
sand-grain sized meteoroids
are entering the atmosphere constantly, essentially every few
seconds in a given region, and thus ionization trails can be found
in the upper atmosphere more or less continuously. When radio waves
are bounced off these trails, it is called
meteor burst
communications.
Meteor radars can measure atmospheric
density and winds by measuring the
decay
rate and
Doppler shift of a meteor
trail.
Colour
The visible light produced by a meteor may take on various hues,
depending on the chemical composition of the meteoroid, and its
speed through the atmosphere. As layers of the meteoroid are
stripped off and ionized, the colour of the light emitted may
change according to the layering of minerals. Some of the possible
colours and the compounds responsible for them are: orange/yellow
(sodium); yellow (iron); blue/green (magnesium); violet (calcium);
and red (silicate).
Sound
There are anecdotal reports of sounds being heard from meteors
entering the Earth's atmosphere. This would seem impossible, given
the relatively slow speed of sound. Any sound generated by a meteor
in the upper atmosphere, such as a sonic boom, should not be heard
until many seconds after the meteor disappeared. However, in
certain instances, for example during the
Leonid meteor shower of 2001, several
people reported sounds described as "crackling", "swishing", or
"hissing" occurring at the same instant as a meteor flare. Similar
sounds have also been reported during intense displays of Earth's
auroras .
Sound
recordings made under controlled conditions in Mongolia in 1998 by
a team led by Slaven Garaj, a physicist at the Swiss Federal Institute of
Technology
at Lausanne
, support the
contention that the sounds are real.
How these sounds could be generated, assuming they are in fact
real, remains something of a mystery.
It has been
hypothesized by some scientists at NASA
as that the
turbulent ionized wake of a meteor interacts with the magnetic
field of the Earth, generating pulses of radio waves. As the
trail dissipates,
megawatts of
electromagnetic energy could be released, with a peak in the
power spectrum at
audio frequencies. Physical vibrations
induced by the electromagnetic impulses would then be heard if they
are powerful enough to make grasses, plants, eyeglass frames, and
other conductive materials vibrate. This proposed mechanism,
although proven to be plausible by laboratory work, remains
unsupported by corresponding measurements in the field.
Frequency of large meteors
- See also:
Planet Earth collision probability with near-Earth
objects
The biggest asteroid to hit Earth on any given day is likely to be
about 40 centimeters, in a given year about 4 meters, and in a
given century about 20 meters. These statistics are obtained by the
following:
Over at least the range from 5 centimeters (2 inches) to
roughly 300 meters (1,000 feet), the rate at which Earth receives
meteors obeys a
power-law distribution
(meaning there is no
typical size in the conventional
sense) as follows:
- N(>D) = 37 D^{-2.7}\
where N(>D) is the expected number of objects larger than a
diameter of D meters to hit Earth in a year. This is based on
observations of bright meteors seen from the ground and space,
combined with surveys of
near Earth
asteroids. Above 300 meters in diameter, the predicted rate is
somewhat higher, with a two-kilometer asteroid (one
million-
megaton TNT equivalent) every
couple of million years — about 10 times as often as the power-law
extrapolation would predict.
Notable meteors
- See also:
Planet Earth collision probability with near-Earth
objects
Perhaps the best-known meteor/meteorite fall is the
Peekskill Meteorite which was filmed on
October 9, 1992 by at least 16 independent videographers.
Eyewitness accounts indicate that the fireball entry of the
Peekskill meteorite started over West Virginia at 23:48 UT (±1
min). The fireball, which traveled in a northeasterly direction had
a pronounced greenish colour, and attained an estimated peak visual
magnitude of -13. During a luminous flight time that exceeded 40
seconds the fireball covered a ground path of some 700 to
800 km.
One meteorite recovered at Peekskill, N.Y., for which the event and
object gained its name, (at 41.28 deg. N, 81.92 deg. W) had a mass
of 12.4 kg (27 lb) and was subsequently identified as an
H6 monomict breccia meteorite. The video record suggests that the
Peekskill meteorite probably had several companions over a wide
area especially in the harsh terrain in the vicinity of
Peekskill.
A large fireball was observed in the skies near Bone Indonesia on
Oct. 8, 2009. This was thought to be caused by an asteroid
approximately 10 meters in diameter. The fireball contained an
estimated energy of 50 kilotons of TNT or about twice the Hiroshima
atomic bomb. No injuries were reported.
A large bolide was reported on November 18, 2009 over South Eastern
California, Northern Arizona, Utah, Wyoming, Idaho and Colorado. At
12:07 am, a security camera at the high altitude W. L. Eccles
Observatory (9600 ft above sea level) recorded a movie of the
passage of the object to the north . Of particular note in this
video is the spherical `ghost' image slightly trailing the main
object (this is likely a lens reflection of the intense fireball),
and the bright fireball explosion associated with the breakup of a
substantial fraction of the object. An object trail can be seen to
continue northward after the bright fireball event.The shock from
the final breakup triggered seven seismological stations in
Northern Utah; a timing fit to the seismic data yielded a terminal
location of the object at 40.286 N, -113.191 W, Alt. 27 km . This
is above the Dugway Proving Grounds, a closed Army testing
base.
History
Although meteors have been known since ancient times, they were not
known to be an astronomical phenomenon until early in the 19th
century. Prior to that, they were seen in the West as an
atmospheric phenomenon, like lightning, and were not connected with
strange stories of rocks falling from the sky.
Thomas Jefferson wrote "I would more easily
believe that (a) Yankee professor would lie than that stones would
fall from heaven."
He was referring to Yale
chemistry
professor Benjamin Silliman' investigation of an 1807 meteorite
that fell in Weston, Connecticut. Silliman believed the
meteor had a cosmic origin, but meteors did not attract much
attention from astronomers until the spectacular meteor storm of
November 1833. People all across the Eastern US saw thousands of
meteors, radiating from a single point in the sky. Astute observers
noticed that
the radiant, as
the point is now called, moved with the stars, staying in the
constellation Leo.
The astronomer
Denison Olmsted made
an extensive study of this storm, and concluded it had a cosmic
origin. After reviewing historical records,
Heinrich Wilhelm Matthias
Olbers predicted its return in 1867, which drew the attention
of other astronomers.
Hubert A.
Newton's more thorough historical
work led to a refined prediction of 1866, which proved to be
correct. With
Giovanni
Schiaparelli's success in connecting the
Leonids (as they are now called) with comet
Tempel-Tuttle, the cosmic origin of meteors
was now firmly established. Still, they remain an atmospheric
phenomenon, and retain their name "meteor" from the Greek word for
"atmospheric".
Gallery
Image:Orionid, pedia.org/wiki/Milky_way Milky Way] and to the right
of
Venus.
Zodiacal
light is also seen at the image.Image:Orionid
meteor.jpg|
OrionidImage:Orionid
meteor1.jpg|
OrionidImage:Two orionids and
milky way.jpg|Two Orionids and
Milky
WayImage:Multi colored Orionid.jpg|Multi-colored
OrionidImage:Orionids and Orion.jpg|OrionidImage:Meteor
trail.jpg|The brightest meteor, a fireball, leaves a smoky,
persistent trail drifting in high-altitude winds, which is seen at
the right-hand side of the image left by
Orionid.Image:Meteoroid meteor
meteorite.gif|Animated illustration of differences between a
meteoroid, meteor and meteorite
See also
References
- Glossary
International Meteor Association
- )
- Povenmire, H. PHYSICAL DYNAMICS OF THE UPSILON PEGASID FIREBLL –
EUROPEAN NETWORK 190882A. Florida Institute of Technology
- Diagram 2: the orbit of the Peekskill meteorite along with
the orbits derived for several other meteorite falls
- MeteorObs Explanations and Definitions (states IAU
definition of a fireball)
- International Meteor Organization - Fireball
Observations
- :156
- usgs.gov - What is a Bolide?
- :133
- The Oxford Illustrated Dictionary. 1976. Second Edition. Oxford
University Press. page 533
- Psst! Sounds like a meteor: in the debate about
whether or not meteors make noise, skeptics have had the upper hand
until now - Now Hear This | Natural History | Find Articles at
BNET.com
- Sound of shooting stars
- Listening to Leonids
- Hearing Sensations in Electric Fields
- Human auditory system response to Modulated
electromagnetic energy.
- Human Perception of Illumination with Pulsed
Ultrahigh-Frequency Electromagnetic Energy
- The Peekskill Meteorite October 9, 1992 Videos
- Brown, P. et al., 1994. Nature, 367, 6524 - 626
- "Meteoritical Bull", by Wlotzka, F. published in "Meteoritics",
# 75, 28, (5), 692, 1994
- [1]Asteroid Impactor Reported over Indonesia
- W.L Eccles Observatory, Nov 18 2009, North Camera
- W.L Eccles Observatory, Nov 18 2009, North West
Camera
- Patrick Wiggins, private communication
- amsmeteors.org The Early Years of Meteor Observations
in the USA
- meteorshowersonline.com The Leonids and the
Birth of Meteor Astronomy
- astroprofspage.com October's Orionid Meteors
External links